This invention relates to a system for measuring and detecting of velocity, turbulence, vorteces and similar irregularities and phenomena in air, including classification of such phenomena. These may comprise the wind velocity, clear air turbulence as well as aircraft induced vortices and turbulence. Detection and measurement as contemplated takes place by combining electromagnetic and acoustic waves.
The invention comprises a system of transmitter and receiver means for electromagnetic waves and transmitter means for acoustic waves, as well as an associated method.
The invention is primarily directed to the measurement of the mentioned phenomena affecting air traffic safety, but may easily be adapted to other fields of use.
This kind of phenomena have been measured before by measuring the scattering and Doppler-shift of electromagnetic beams when affected by the disturbances in the air. An example of this is given in the international patent application WO 93/19383 using electromagnetic transmitter and receiver means. The electromagnetic beam is transmitted at a microwave frequency and is scattered by disturbances in the air. The receiver means are positioned at a bistatic location and directed so as to cover a volume of air also covered by the transmitter, and to receive signals with a (preferrably low) scattering angle.
Scattering of electromagnetic waves by acoustic waves is discussed in Appl. Sci. Res. Section B, Vol. 6, 1957 by A. Tonning: "Scattering of electromagnetic waves by an acoustic disturbance in the atmosphere". The publication discusses the cases of spherical and plane acoustic waves.
This invention is based on the fact that acoustic waves generate disturbances in the dielectric constant in the air by changing its density. These disturbances are affected by wind and other phenomena in the air, and also scatter the electromagnetic energy. By controlling the acoustic signal and/or the electromagnetic energy the scattering may be controlled, and the received signal may then be use to measure the other disturbances in the air. A system similar to this is described in U.S. Pat. No. 4,351,188 in which radio waves are transmitted towards a spherical acoustic wavefront. The electromagnetic wave is reflected and focused by the acoustic wavefront towards electromagnetic receivers on the ground. A two-dimensional array of receivers is used to measure the position to which the electromagnetic wave is focused. This position and the time delay give an indication of the wind velocity at a given altitude. A disadvantage in this system is the limitations in the wind directions that can be measured. To get a reflection focused inside the receiver array the wind must blow along the direction from the electromagnetic transmitter, across the acoustic transmitter towards the electromagnetic receivers. To get a complete wind profile four electromagnetic and acoustic transmitters are used. Another disadvantage with this solution is that the transmitters and receivers need to be positioned close to each other, which limits the possibility for using existing components at the site in which the measuring is to be performed.
The present invention avoids this problem by generating a grating in the dielectric constant of the air and measuring the fluctuations and movements of this grating to get a complete wind profile in a chosen air volume. The novel and specific features according to the invention are set out more completely in the claims.
According to the invention the acoustic signal is essentially periodic. This will generate a generally periodic disturbance in the dielectric constant of the air. In this way a grating may be generated, moving at the speed of sound, with a spatial wavelength depending on the wavelength (and thereby frequency) of the acoustic energy. The grating will satisfy the Bragg conditions for certain combinations of acoustic and electromagnetic frequencies and thereby reflect part of the electromagnetic energy towards the receivers.
The different movements in the air will affect the grating in different ways. Air moving parallel to the direction of propagating acoustic energy will change the density of the grating, and air moving perpendicular to the acoustic energy will displace the grating. By measuring parameters like the acoustic or electromagnetic frequencies, or displacement or angle of arrival of the received electromagnetic wavefronts the air velocity and direction may be found.